Data processing system employing quench simulation for enabling accurate computation of sample activity levels in liquid scintillation spectrometry

ABSTRACT

A data processing system for scintillation spectrometers of the type for measuring activity levels of samples containing radioactive isotopes and subjected to varying degrees of quench, including methods and apparatus for imposing a controlled simulated quench condition in each sample which, when added to the actual internal quench condition of the sample, creates an effective quench condition equal to a known predetermined actual quench condition for which counting efficiency is known with a high degree of accuracy. whereby true activity levels can be accurately computed in decay events per minute without incurring statistical errors inherent in interpolation and extrapolation techniques employed with conventional quench correlation data. Various methods and apparatus are described for creating a controlled simulated quench for each sample, together with an automatic computational system which permits direct display of activity levels in units of decay events per minute.

United States Patent Packard [72] Inventor: Lyle E. Packard, Hinsdale, Ill.

[73] Assignee: Packard Instrument Company, Inc.,

Downers Grove, Ill.

[22] Filed: April 14, 1967 [21] App1.No.: 630,892

[52] US. Cl ..250/ 106 SC, 250/7l.5 R, 250/83 C, 250/205, 250/207 [51] Int. Cl ..G01t 1/20 [58] Field of Search..250/l06 SC, 71.5 R, 205, 83 C, 250/83 D, 207

[56] References Cited UNITED STATES PATENTS 8/1963 Fite ..250/71.5 3/ 1964 Schrenk et a1 ..250/205 6/1965 Packard et a1 ..250/71.5 4/1968 Nather ..250/7 1 .5

OTHER PUBLICATIONS Baille, L. A., Determination of Liquid Scintillation Aug. 29, 1972 Counting Efficiency, by Pulse Weight Shift, Intemational Journal of App. Radiation Isotopes, 1960, Vol.

Primary Examiner-James W. Lawrence Assistant ExaminerMorton J. Frorne Attorney-Hubbard, Leydig, Voit & Osann Wolfe [57] ABSTRACT A data processing system for scintillation spectrometers of the type for measuring activity levels of samples containing radioactive isotopes and subjected to varying degrees of quench, including methods and apparatus for imposing a controlled simulated quench condition in each sample which, when added to the actual internal quench condition of the sample, creates an effective quench condition equal to a known predetermined actual quench condition for which counting efficiency is known with a high degree of accuracy. whereby true activity levels can be accurately computed in decay events per minute without incurring statistical errors inherent in interpolation and extrapolation techniques employed with conventional quench correlation data. Various methods and apparatus are described for creating a controlled simulated quench for each sample, together with an automatic computational system which permits direct display of activity levels in units of decay events per minute.

29 Claims, 19 Drawing Figures PATENTED 3,688,120

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I) v WW/W DATA PROCESSING SYSTEM EMPLOYING QUENCH SIMULATION FOR ENABLING ACCURATE COMPUTATION OF SAMPLE ACTIVITY LEVELS IN LIQUID SCINTILLATION SPECTROMETRY CROSS REFERENCE TO RELATED APPLICATIONS Robert E. Cavanaugh, Jr., Ser. No. 541,721, filed Apr. 11, 1966. Stanley M. Bristol, Ser. No. 629,462, filed Apr. 10, 1967. Robert E. Cavanaugh, Jr., Ser. No. 630,891 filed Apr. 14, 1967.

BACKGROUND OF THE INVENTION The present invention relates in general to liquid scintillation spectral analysis of test samples containing unknown isotopes disposed in a liquid scintillator and, more particularly, to spectral analysis equipment and techniques which may be wholly automatic in operation, which virtually eliminate the effects of quenching as a source of error in determining true activity levels of test samples, and which readily permit direct display of such true activity levels, either visually or in printed form, in units of decay events per minute (dpm). In its principal aspects, the invention is concerned with an improved data processing system, as well as with improved methods and apparatus for simulating a controlled quench condition for all test samples, irrespective of whether the latter are actually unquenched or quenched and irrespective of the degree to which such samples may actually be quenched, whereby the effective quench condition of the sample (viz., the amount of actual quench internally of any given sample plus the simulated quench condition superimposed thereon externally of the sample) converges to a known selected value for which instrument counting efficiency is known with a high degree of accuracy.

Modern apparatus for detecting and measuring radioactivity has reached an unusually high state of development with systems currently available which offer unusual sensitivity to low energy radiation, as well as various options of full automation, semi-automation, or the more economical manual operating version. In a relatively few years, great strides have been made towards improving the preciseness and accuracy of counting efiiciency in compliance with the very stringent requirements of users of this highly technical and sophisticated equipment. However, certain problems have continued to plague both the manufacturers and users of such equipment. A particularly prevalent and vexing problem has been the error introduced into computations of true sample activity levels because of a phenomenon commonly encountered with liquid scintillation samples known as quenching. Stated very simply, this phenomenon results in attenuation of light scintillations within the samples, thus significantly affecting the statistical accuracy of the equipment which determines activity levels based upon the number and energy of such light scintillations, the latter being counted over known units of time and being proportional in energy to the energy of the decay events which produce them.

Many efforts have heretofore been made to minimize and, preferably, to eliminate, the errors which result from the quench phenomenon, some of which have completely failed and others of which have met with varying degrees of success and acceptance--however, notwithstanding all such efforts, the problem has remained as a source of error, which ofttimes amounts to significant error in the computation of true activity levels.

One principal effort that has heretofore been made towards minimizing the quench problem has been that of development of various constituents which make up the sample and which are as free of quench characteristics as possible. Such constituents include, without limitation thereto, scintillation substances, solvents, and the material from which the light transmissive sample vial is made. However, perfect light transmitters completely devoid of quench characteristics are simply not available, and even if they were, the problem would remain since the test specimen itself may, and often will, contain quench materials such, for example, as blood or urine, which tend to attenuate the light because of their color. Moreover, unless the detection system is maintained in a completely enclosed atmosphere of an inert gas such as argon, quench can occur simply because of the presence of air.

Faced with the seeming impossibility of eliminating the quench phenomenon as a source of error, numerous efforts have been made to cope with the problem by providing methods and apparatus for compensating for such errors. Typical systems which are currently employed and which have found great acceptance today by people employing this sophisticated equipment include systems in which an external standard source which emits highly-penetrating radiations is placed adjacent the sample in the detection chamber during a portion only of its overall counting cycle. Light scintillations occurring in the sample are then counted during at least two discrete intervals, during one of which the scintillations are created only by the isotope in the sample and during the other of which the scintillations are created by the composite effect of the isotope and the external standard. Suitable electronic equipment is provided for separating the pulses from the two sources on the basis of their different energy levels and, therefore, those which are counted primarily from the external standard provide a fairly accurate indication of the degree of quenching present in the sample since the counting efficiency for such external standard is known or can be readily ascertained by use of an unquenched standard sample. Typical systems of this type are described in detail in Lyle E. Packard US. Pat. No. 3,188,468, issued June 8, 1965, as well as in the aforesaid Cavanaugh application Ser. No. 541,721, filed Apr. 11, 1966, both of which are assigned to the assignee of the present invention.

Both of the aforementioned prior systems are of the type which are commonly referred to as extemal standardization systems and both represent basic and significant improvements over earlier known systems described therein, such as internal standardization and channels ratio systems. In effect, however, all of these prior systems have had certain aspects which are common to one another, a principal one of which is that the'measured quench correlation parameter (e.g., net external standard count, external standard ratio, channels ratio, etc.) generally provides an indication of the degree of quench present in the sample, which indication must then be compared with a previously prepared quench correlation curve in order to determine the counting efficiency. Once knowing the counting efficiency, the counts per minute (cpm) measured for the isotope being analyzed can be divided by counting efficiency to determine activity level in decay events per minute (dpm). Unfortunately, however, the quench correlation curve itself differs widely from instrument to instrument, from isotope to isotope, from channel to channel, with sample volume, and with other variable conditions. Consequently, it has heretofore been necessary to prepare many of such curves, the preparation of each one of which has been time consuming, expensive, and subject to numerous human errors. Moreover, once the curves are prepared, it is necessary that the measured quench correlation data be compared with them in order to determine counting efficiency, thus introducing even further danger of human error.

Even more significant, however, has been the fact that while such a correlation curve can be prepared, it is only as accurate as the number of points which actually define the curve. It has been established that such points simply do not fall on the straight line, or even on a smoothly curved line--quite to the contrary, the points will be non-uniformly distributed in an unpredictable random pattem which only generally defines the correlation curve. Consequently, even when the technician uses extreme care in his computations, he has been forced to extrapolate or interpolate between known points and, since the extrapolated or interpolated data can vary significantly from the actual data, the computed efficiency can still vary greatly from actual efficiency with maximum errors on the order of up to percent and average errors on the order of up to 2 percent being common, dependent upon the number of differently quenched standard samples selected to prepare the quench correlation curve.

Errors of the foregoing magnitude are simply not acceptable to the highly trained technical personnel who use this general type of equipment. Indeed, such errors are highly objectionable, and the more so in view of the high state of sophistication that the overall art has reached. However, these errors have heretofore been tolerated only because the prior systems briefly described above, and described in considerably greater detail in the aforesaid Packard U.S. Pat. No. 3,188,468 and Cavanaugh application Ser. No. U.S. Pat. No. 3,499,149, have represented the best available solutions to the problem up until the present date.

Continued efforts have, however, been made towards providing a more satisfactory solution to the problem. For example, it has been suggested that true activity level for a sample can be computed simply by dividing the measured variable quench correlation parameter (e.g., external standard ratio, net external standard count, channels ratio, etc.) into the measured value in counts per minute (cpm) for the sample undergoing analysis. This suggestion, however, is not satisfactory for many reasons. First, it assumes that the quench correlation curve is a straight line, which it is not. Secondly, it fails to take into account the non-uniform random distribution of points which define such a curve. Therefore, even were the curve a straight line or substantially a straight line, errors of the same general magnitude as described above would still be experienced. The fact that the quench correlation curve is not a straight line actually adds to the magnitude of such errors with the result that errors on the order of up to 25 percent can be, and have been, experienced.

It has also been proposed that the problem can be resolved by adjusting in any of various known manners, overall system gain so as to restore the measurable quench correlation parameter to a value indicative of an unquenched sample, and thereafter, analyzing the sample as if it were unquenched. Again, however, such a proposed solution is no solution at all since the gain correlation curves do not coincide with nor follow the quench correlation curves and, consequently, the magnitude of error can be and often will be, even greater than that experienced with the interpolation/extrapolation techniques referred to above.

It is a general aim of the present invention to provide an improved data processing system which overcomes the foregoing disadvantages and which is characterized by its reliability and rapidity of operation. In this connection, it is an object of the invention to provide improved radioactivity spectrometry methods and apparatus which permit the determination of activity levels for test samples having any of a wide range of different quench characteristics with virtually unlimited accuracy, yet wherein this is accomplished by the utilization of quench correlation data based upon the measurement of only a relatively few, and indeed, in some instances, only one, known standards or standard. While not so limited in its application, the invention will find especially advantageous use when the measured variable parameter of such quench correlation data takes the form of net external standardization ratios, although it can also be used in connection with other measurable variable parameters which also provide an indication of the degree of quenching such, merely by way of example, as channels ratios, or net external standard counts.

As a consequence of attaining the foregoing general objective of the invention, it has been found that ancillary benefits achieved are that the danger of human error in both the preparation and reading of quench correlation data is greatly reduced; the versatility of such data is significantly increased; and the time and cost required to prepare such data are held to a minimum.

In another of its important aspects, it is an object of the invention to provide improved methods and apparatus suitable for use in radioactivity spectrometry applications which permit highly accurate determination of sample activity levels in terms of decay events per minute (dpm), and where such information can be printed or read directly without requiring the technician to perform close and tedious comparisons of detected or measured data with quench correlation curves.

It is a related object of the invention to provide improved spectrometry methods and apparatus which will greatly facilitate and speed up the quantitative determination of activity levels in terms of decay events per minute (dpm), yet wherein this is not only accomplished without sacrificing accuracy of the computations but, to the contrary, wherein the measurements are, on an average, considerably more accurate than has heretofore been feasible on a commercial basis.

It is one of the principal objectives of the present invention to provide novel spectrometry methods and apparatus for determining activity levels of test samples wherein provision is made for automatically compensating for errors attributable to quenching so as to permit direct display of activity levels in units of decay events per minute (dpm), either visual display or printed display, all irrespective of the amount of quenching that may be present in any given sample.

Another general aim of the present invention is the provision of novel and improved methods and apparatus suitable for use in computing true activity levels, corrected for quench errors, of test samples in units of decay events per minute (dpm) which are characterized by their accuracy and by their complete elimination of any need to visually or automatically interpolate or extrapolate between known points on a quench correlation curve, which interpolation or extrapolation has heretofore been essential and which has inherently introduced significant errors into activity level computations.

A further object of the invention is the provision of novel methods and apparatus for producing simulated quench conditions for each test sample undergoing examination so as to bring the effective quench level of the sample (the effective quench level consisting of the actual internal quench level of the sample plus the superimposed external simulated quench level) to a known and selectable level for which counting efficiency is accurately known, yet wherein this is accomplished without any noticeable loss in statistical counting accuracy.

It is a more specific object of the invention to provide novel methods and apparatus for statistically modulating signals representative of decay events in each test sample so as to simulate the presence of a quenching agent in the sample, and for adjusting the modulating signals so as to cause the effective quench level of the sample to converge upon a particular preselected quench level for which the counting efficiency is known with a high degree of accuracy. In this connection, it is an object of the invention to simulate quenching for all test samples, whether the latter are quenched or unquenched, and irrespective of the degree of quenching that may be present, so as to bring the particular variable quench correlation parameter being measured (e.g., external standard ratio; net external standard count; channels ratio; etc.) from its actual value for any given sample to a particular preselected simulated value where counting efficiency is known with a high degree of accuracy and which is at or somewhat below the value for such parameter for the most quenched sample that would normally be encountered by the technician.

It is a more detailed object of the invention to provide novel methods and apparatus for modulating the photon energy given off by the scintillating medium in the region between the scintillator and the photon-toelectron energy transducer, and thus to simulate a quench condition for the sample equivalent to that produced by the presence of an actual quenching agent in the sample itself, which simulated quench condition is automatically controlled so as to adjust the effective quench level for any given sample (irrespective of its actual quench level) to a preselected point where counting efficiency is known with a high degree of accuracy.

An ancillary object of the invention is the provision of novel methods and apparatus for establishing quench correlation data rapidly, economically, and with a high degree of accuracy, yet without requiring any advance preparation of standards having known and different quench conditions, and without requiring repetitive manipulation of such standards either manually or automatically, and wherein an unlimited number of simulated quench conditions can be produced using only a single unquenched standard.

Other objects and advantages of the invention will become apparent as the following description proceeds, taken in conjunction with the accompanying drawings, in which:

FIG. 1 is a fragmentary side elevational view, partly in section, depicting an exemplary radiation detection chamber and elevator mechanism suitable for processing samples in accordance with the present invention, the apparatus here being depicted with the elevator mechanism in the down or sample loaded position with the sample to be analyzed interposed between a pair of light transducers;

FIG. 2 is an enlarged fragmentary side elevational view, partly in section and partly in diagrammatic form, here showing a conventional apparatus for selectively positioning and recirculating external standard radioactive source material between a first position remote from and shielded from the detection chamber and a second position adjacent a sample disposed in the detection chamber;

FIG. 3 is a fragmentary schematic wiring diagram of the control components utilized for positioning standard radioactive source material in a selectable one of two positions in accordance with the particular cycle of operation determined by the mode program control, the latter being depicted partially in block and partially in diagrammatic form;

FIG. 4 is a schematic wiring diagram, partly in block form, here depicting an exemplary control system for operating the elevator mechanism shown in FIGS. 1 and 2 together with a conventional electrical system which accepts, counts and records the output of the radiation detector while at the same time providing certain control inputs in accordance with the present invention for the exemplary computer program control shown in FIG. 13;

FIG. Sis an elevational view of a typical master control panel suitable for use with apparatus embodying the features of the present invention and capable of carrying out the methods of the present invention;

FIG. 6 is an elevational view, partly in diagrammatic form, of an exemplary auxiliary control panel by which the technician may selectively dial in predetermined efficiency values, or numerical representations thereof, for one or more isotopes in different pulse height analyzing channels so as to enable direct display of isotope activity level in units of decay events per minute (dpm) in accordance with the present invention;

FIG. 6a is an enlarged fragmentary elevational view of one of the BCD (binary coded decimal) decade dial switches shown in FIG. 6;

FIG. 7a is a graphic representation of typical pulse height spectra characteristic of two beta emitting isotopes of different energy levels and illustrating particularly the principle of balance point operation for the lower energy isotope as well as the effect of quenching on both the lower energy isotope and the higher energy isotope;

FIG. 7b is a graphic representation of typical pulse height spectra similar to FIG. 7a but here indicating the abscissa, which is sealed in units of pulse height, on an attenuated scale so as to shift the pulse height spectra downwardly and to the left with the higher energy isotope being illustrated in balance point within its pulse height analyzing channel;

FIG. 8 is a graphic representation of a typical smooth quench correlation curve (in which the abscissa is scaled in units of net external standard ratio as the measured variable quench correlation parameter indicative of quenching), which smooth curve is commonly used for determining the counting efficiency of a liquid scintillation counting system and which has heretofore led to significant statistical errors in computations of true activity levels for isotopes in test samples, here emphasizing particularly the non-uniform random distribution of actual measured points which define such a curve;

FIG. 8a is a graphic representation similar to FIG. 8 with the continuous smooth curve deleted and emphasizing particularly specific designated intercept points for which counting efficiency is known with a high degree of accuracy;

FIG. 9 is a graphic representation of a slightly different type of quench correlation curve which is somewhat similar to the curve shown in FIG. 8 but in which the abscissa is scaled in units of net external standard count as the measured variable quench correlation parameter indicative of the degree of quenching;

FIG. 10 is a graphic representation similar to FIGS. 8 and 9 but in which the abscissa is scaled in units of normalized channels ratio as the measured variable quench correlation parameter which is indicative of the degree of quenching;

FIG. 1 l is an enlarged elevational view, partly in section and partly in diagrammatic and block form, somewhat similar to FIG. 1 but with the housing depicted in phantom and certain parts removed for purposes of clarity, here illustrating a radiation detection chamber made in accordance with one exemplary form of the invention and which is suitable for simulating controllable quench conditions for test samples disposed in the chamber;

FIG. 12 is a perspective view on a reduced scale, partially in diagrammatic and block form, depicting the same general organization of components as is shown in FIG. 11, but here illustrating particularly a slightly different type of detection chamber suitable for simulating quench conditions in test samples in accordance with a modified form of the invention;

FIG. 13 is a block diagram of an exemplary data processing system suitable for use in conjunction with the present invention, here illustrating particularly in block form a conventional time-shared digital computer together with the appropriate computer inputs and outputs;

FIG. 14 is a block diagram of exemplary control circuitry suitable for use in conjunction with the present 8 invention for providing incremental changes in the quench simulation modulating signal so as to cause the effective quench level of the sample to converge upon a selectable predetermined quench level where counting efficiency is known with a high degree of accuracy;

FIG. 15 is a schematic wiring diagram of an exemplary arbitrary function digital-to-analog converter which is suitable for converting the output signals from the circuit of FIG. 14 to a useable control signal for modulating simulated quench conditions; and,

FIG. 15a is schematic wiring diagram illustrating details of one of the illustrative transistor circuits shown in FIG. 15.

While the present invention is susceptible of various modifications and alternative forms, specific embodiments thereof have been shown by way of example in the drawings and will herein be described in detail. It should be understood, however, that it is not intended to limit the invention to the particular forms disclosed, but, on the contrary, the intention is to cover all modifications, equivalents and alternatives falling within the spirit and scope of the invention as expressed in the appended claims.

THE ENVIRONMENT OF THE INVENTION Before discussing the present invention in detail, it may be helpful to first briefly consider the prior art background or environment within which the present invention finds especially advantageous application. In radioactivity measurements, it is the ultimate objective of most technicians to determine the activity level of isotopes which may be either singly present or present in multiple combinations within test samples, such activity level being expressed generally in units of decay events per unit time, e.g., decay events per minute (dpm). Thus, the quantity of a particular isotope present in a test sample is, in general, proportional to the rate of decay events produced by that isotope, such rate being termed the activity level of the source. As a generalization, the decay events or radiation emana tions from a radioactive source are, for purposes of measurement or counting, converted into light scintillations which are generally proportional in photon energy to the energy of the decay event which caused them. The light scintillations are then generally converted into corresponding voltage or current pulses which are generally proportional to both the light scintillation and the decay event which caused the particular pulse. Such pulses are then discriminated on the basis of their amplitude, and then counted. The pulses may be counted for a predetermined time (termed preset time operation) or, alternatively, a predetermined number of pulses can be counted and the time required to reach such predetermined count measured (termed preset count operation). Generally, the ratio of counted pulses to the elapsed time period is indicative of the activity level of the sample.

a. General Organization and Operation Of An Exemplary Sample Processing Apparatus Referring now to FIGS. 1 and 2 conjointly, there has been illustrated an exemplary automatic sample processing apparatus, generally indicated at 20, which is intended to transfer a plurality of samples one at a time in seriatim order to and from a detection station. To this end, the exemplary apparatus 20 includes an elevator and detector mechanism, generally indicated at 21, which is positioned beneath a support table 22 (FIG. 1) on which one or more sample vials 24 are positioned both prior to and subsequent to transfer to a counting or detection station. As the ensuing description proceeds, those skilled in the art will appreciate that the particular means employed for conveying samples 24 to and from a point of alinement with the elevator mechanism 21 is immaterial to the present invention. Thus, it will be understood that the samples 24 may be carried in rotatable trays in the manner described in greater detail in both the aforesaid Packard US. Pat. No. 3,188,468 and Packard et al. US. Pat. No. 3,257,561, issued June 21, 1966 and assigned to the assignee of the present invention. Alternatively, a plurality of successive sample vials may be conveyed to a point of alinement with the elevator mechanism 21 by means of an endless conveyor having separate supports for the various vials. And, of course, it will be understood that successive sample vials may be manually placed on and removed from elevator apparatus 21. Moreover, while there has herein been illustrated and will be described a power driven elevator mechanism 21 for conveying successive samples 24 into and out of a detection chamber, the elevator need not be automatic and could take the form of a manually operated elevator of the type illustrated in one of the fomrs of the invention disclosed and claimed in Robert E. Olson US. Pat. No. 3,198,948, issued Aug. 3, 1965 and assigned to the assignee of the present invention. Indeed, the present invention will also find advantageous use in more rudimentary forms of radiation detection devices which are completely manual in operation and which do not employ any type of elevator, the samples being manually positioned within and removed from the detection chamber.

However, to facilitate an understanding of the present invention, the general organization and operation of the elevator and detector mechanism 21 will be briefly described hereinbelow. Those interested in a more complete operational and structural description of the mechanism 21 are referred to the aforesaid Packard and Olson patents.

Referring to FIG. 1, it will be noted that the elevator and detector mechanism 21 includes a base assembly 25 which houses a pair of light transducers, for example, photomultipliers PMT 1 and PMT 2 disposed on opposite sides of a vertical elevator shaft 26. Mounted within the elevator shaft 26 is an elevator 28 having a platform 29 at its upper end for supporting one of the radioactive test samples 24 and transporting the sample downwardly into the elevator shaft where it is alined between the photomultipliers PMT 1 and PMT 2. Each sample 24 may simply comprise a light transmissive vial or other suitable light transmissive container within which is placed a liquid scintillator and a radioactive isotope or isotopes to be measured. Thus, as the isotope or isotopes undergo decay events, light scintillations are produced in the light scintillator, and such scintillations are then detected by the photomultipliers which produce electrical output signals in the form of voltage or current pulses corresponding'to each light scintillation detected. At the completion'of the counting cycle the elevator 28 is returned upwardly so as to eject the sample 24 from the elevator and detector mechanism 21. A shutter mechanism 30 is mounted on the upper end of the base assembly 25 for the purpose of preventing erroneous output signals from the photomultipliers resulting from environmental light. At the same time, the base assembly 25 is formed of suitable shielding material such, for example, as lead, which serves to minimize the amount of environmental ionizing radiation causing light flashes in either the scintillation medium or the photomultipliers.

In order to insure that the shutter mechanism 30 is opened and closed in timed relationship with vertical movement of the elevator 28, the two devices are interconnected and actuated by a common reversible drive motor M1 (FIGS. 1 and 4). While not illustrated in detail, the shutter mechanism 30 comprises a plurality of movable shutter blades which are interleaved with a plurality of fixed shutter blades, the latter having apertures therein alined with the elevator shaft 26 and with an aperture 31 formed in the tablelike support 22. The arrangement is such that when the movable blades are pivoted about a pivot point (not shown), they swing between limit positions to selectively open and close the upper end of the elevator shaft 26.

To effect such pivotal blade movement, the movable shutter blades are rigidly secured to a stub shaft (not shown) in a manner more fully described in the aforesaid Olson patent. Suffice it to say that the stub shaft is rigidly secured to the upper end of a generally flat, depending shutter actuating shaft 32 (FIG. 2) having a twisted portion 34 intermediate its ends. The lower end of the actuating shaft 32 is received within a tubular drive shaft 35 (FIG. 2) the latter being coupled adjacent its lower end to the elevator 28 by means of a bracket 36. A pair of dowel pins (not shown) or similar cam means extend transversely through the tubular drive shaft 35 in closely spaced surrounding relation to the shutter actuating shaft 32.

The arrangement is such that as the drive shaft 35 starts to move vertically upward, force is transmitted through the bracket 36 and the elevator 28, thus driving the latter upwardly to unload the sample 24. Just prior to the time that the sample 24 reaches the shutter mechanism 30, the dowel pins or similar cam means traverse the twisted portion 34 of the shutter actuating shaft 32, rotating the latter about its own vertical axis and pivoting the movable blades of the shutter mechanism out of the path of vertical movement of the elevator 28. During a sample loading cycle, the dowel pins or similar cam means serve to cam the shutter actuating shaft 32 in the opposite direction immediately after the new sample 24 passes through the alined apertures in the shutter mechanism and the table, thus swinging the movable blades to the closed position shown in FIG. 1.

To efiect vertical movement of the drive shaft 35 and the elevator 28 for the purpose of introducing samples 24 into and ejecting such samples out of the elevator shaft 26, the drive shaft 35 is drivingly coupled to a conventional reversible motor M1 (FIGS. 1 and 4). The particular means employed for coupling the motor to the drive shaft may vary and have not been illustrated or described in detail. Those interested in a more complete description 'are referred to the aforesaid Packard and Olson patents. It should suffice to state for the purpose of an understanding of the present invention that the motor M1 is coupled to the drive shaft 35 in the exemplary apparatus by means of cables diagrammatically indicated at 38 in FIG. 4. The arrangement is such that when the motor M1 is driven in one direction, the cables 38 are paid in and out so as to raise the elevator mechanism 21. Conversely, when the motor is driven in the opposite direction, the cables are paid in and out in the opposite direction, thus lowering the elevator mechanism 21.

The energizing circuit for the motor M1 includes a lower limit switch LS1 (FIGS. 2 and 4) which is mounted on the frame of the sample handling apparatus 20 in a position to have its actuator LS1 depressed by'a laterally projecting flange 39 mounted on the lower end of the elevator when the latter is in a down position with the sample 24 carried thereon alined between the photomultipliers PMT 1, PMT 2. Depression of the actuator LS1 serves to deenergize the motor M1 and the apparatus is then ready for a counting cycle. A second limit switch LS2, included in a second energizing circuit for the motor M1, is mounted on the frame of the apparatus 20 in position to have its actuator LS2,, depressed by the flange 39 when the elevator arrives at its uppermost limit position with the sample 24 carried thereon having been ejected from the elevator shaft 26. Thus, the limit switch LS2 serves to deenergize the motor M1 when the elevator reaches its uppermost limit position.

Referring now to FIGS. 1, 2 and 4 conjointly, a brief description of a typical sample unloa and sample load cycle of operation will be set forth. Assuming that the exemplary elevator mechanism 28 is in its down position and that the sample vial 24 positioned in the detection chamber has undergone a complete counting operation for determining the activity level of the radioactive source therein, the technician is now ready to remove the particular sample 24 from the detection chamber between the photomultipliers PMT it 1, PMT 2 and to substitute therefor a new sample 24. Considering for the moment, a semi-automatic operating cycle, it is merely necessary for the technician to press the UNLOAD button on the left hand portion of the master control panel (FIG. When this is done, an energizing circuit will be completed from the terminal L2 of a suitable a-c source (not shown) through any suitable circuit (not shown) in the Mode Program Control 40, and from thence through the Change Sample terminal 41, the normally closed RUN contacts of the upper limit switch LS2, and through the UNLOAD terminal of the elevator motor M1, the latter being coupled to the terminal L1 of the a-c source. Under these conditions, the motor Ml will be energized and will start to rotate so as to raise the drive shaft 35 and the elevator 28. As the elevator 28 starts upwardly, the flange 39 which is integral with the bracket 36 will release the actuator LS1, of the lower limit switch LS1, thus permitting the latter to return to its normal condition with the RUN contacts closed and the STOP" contacts open. As the elevator 28 approaches its upper limit position (the shutter mechanisms 30 having been opened by coaction of the shutter actuating shaft 32 and the drive shaft 35), the flange 39 engages the actuator LS2, of the upper limit switch LS2, thus shifting the latter to open its normally closed RUN contacts and close its normally open STOP contacts. When this occurs, the elevator motor M1 is deenergized and the sample vial 24 is in the sample ejected position.

During a semi-automatic operating cycle, the technician simply replaces the ejected sample vial with a new sample vial and then depresses the LOAD mode selector switch shown in the left hand portion of FIG. 5. On the other hand, in a completely automatic cycle of operation, closure of the STOP contacts of the upper limit switch LS2 could, through suitable circuitry not shown but described in greater detail in the aforesaid Packard et al. U.S. Pat. No. 3,257,561, cause energization of the indexing mechanism for automatically moving the next sample into place. In either case, when the new sample is in place an energizing circuit is completed from the terminal L2 of the a-c source through the Mode Program Control 40, its Sample Changed terminal 42, the normally closed RUN contacts of the lower limit switch LS1, and then through the LOAD terminal of the elevator motor back to the a-c terminal L1. The motor now runs in the opposite direction to again return the elevator 28 to its lowermost position. At the same time, the shutter mechanism 30 is closed as the drive shaft 35 moves downwardly and the cam means therein traverses the twisted portion 34 of the shutter actuating shaft 32. When the elevator 28 reaches its lowermost limit position, the cam actuator or flange 39 again engages and depresses the actuator LS1 of the lower limit switch LS1, thus breaking the RUN contacts and making the STOP contacts thereof. The motor M1 is again deenergized and the apparatus is now ready for another count cycle. Closure of the STOP contacts of the lower limit switch LS1 is efi'ective to create a control signal from the terminal Ll of the a-c source to the Elevator Down terminal 43 of the Mode Program Control 40, thus signaling the latter that the apparatus is in condition for automatic initiation of the next counting cycle.

b. Automatic External Standardization As stated above, the aforesaid Packard U.S. Pat. No. 3,188,468 discloses and claims various forms of procedures and apparatus for automatically subjecting successive samples to two separate counting cycles, during one of which the sample to be counted is exposed to a known quantity per unit time of radiant energy emanating from either an internal or an external standard. Generally stated, external standardization techniques are based upon a phenomenon known as Compton Scatter, a phenomenon wherein the interactions that occur between penetrating radiation and electrons that comprise part of the test sample, produce electrons in the liquid scintillator having an energy spectrum similar in shape to that produced by a beta emitter. The Compton Scatter phenomenon is well known and need not be described in detail. Those interested in such a detailed description are referred to the aforesaid copending Cavanaugh application Ser. No. 541,721. Briefly, however, and with reference to FIG. 2, it will be observed that a standard source of penetrating radiation, here a compound source generally indicated at 44, has been illustrated at a position located exterior of and in proximity to the test sample 24 disposed in the detection chamber. As is 

1. Apparatus for simulating quenching in test samples containing one or more radioactive isotopes disposed in a liquid scintillator comprising a light transducer, means for positioning a sample adjacent said transducer in light transmissive relationship therewith so as to establish a sample-transducer combination which permits counting of light scintillations occurring in the sample, and means for controllably altering the counting statistics of said sample-transducer combination so as to simulate quenching in said sample.
 2. Apparatus as set forth in claim 1 further characterized in that said means for altering counting statistics comprises means for reducing the light input to said transducer.
 3. Apparatus as set forth in claim 2 further characterized in that said light input reduction means comprises means for moving said transducer away from said sample in controlled increments.
 4. Apparatus as set forth in claim 2 further characterized in that said light input reduction means comprises a mechanical shutter interposed between said transducer and said sample, and means for controllably adjusting said shutter to vary said light input.
 5. Apparatus as set forth in claim 4 further characterized in that said mechanical shutter comprises a variable iris lens.
 6. Apparatus as set forth in claim 2 further characterized in that said light input reduction means comprises a polarizing lens.
 7. Apparatus as set forth in claim 2 further characterized in that said light reduction means comprises means for altering the efficiency of the liquid scintillator.
 8. Apparatus for simulating quench conditions for a sample containing at least one radioactive isotope disposed in a liquid scintillation medium comprising, in combination, means for measuring a selected quench correlation parameter for the sample, means for establishing a variable simulated quench modulating signal having a value which is a function of the measured quench correlation parameter, and means for controlling the variable simulated quench modulating signal so as to affect counting statistics to cause a measured selected quench correlation parameter for the sample to shift towards a fixed preselected point.
 9. Apparatus for simulating quench conditions for a group of successive samples having diverse internal true quench levels and each containing at least one radioactive isotope disposed in a liquid scintillation medium comprising, in combination, means for establishing a fixed preselected quench correlation parameter for which counting efficiency is accurately known and which is below the true measurable quench correlation parameter for the most quenched sample in said group, means for measuring the quench correlation parameter for each sample, means for establishing a variable simulated quench modulating signal having a value which is a function of the measured quench correlation parameter for each sample, and means for controlling the variable simulated quench modulating signal so as to affect counting statistics to cause the measured quench correlation parameter for all of said samples to converge to the fixed preselected quench correlation parameter for the most quench sample in said group.
 10. Apparatus as set forth in claim 9 further characterized in that said quench correlation parameter is net external standard ratio.
 11. Apparatus as set forth in claim 9 further characterized in that said quench correlation parameter is net external standard count.
 12. Apparatus as set forth in claim 9 further characterized in that said quench correlation parameter is channels ratio.
 13. A data processing system for determining activity levels of test samples containing one or more radioactive isotopes in units of decay events per unit time and wherein counting efficiency for the system is known only at n discrete fixed points for any given isotope and wherein a measurable quench correlation parameter R for the sample is known at each of said points, said system comprising, in combination, first means for storing a numerical representation of counting efficiency for a known value of the parameter R corresponding to one of the n discrete points, second means for measuring the parameter R for each sample, third means for comparing the measured parameter R for the sample with the known value of parameter R, fourth means for simulating quenching for each sample, fifth means for controllably varying said fourth means to cause the measured parameter R to converge upon the known value of the parameter R, sixth means for counting light scintillations per unit time created in the sample by isotope decay events therein and for producing a numerical representation of counts per unit time, and seventh means for dividing the stored numerical representation of counting efficiency into the numerical representation of counts per unit time which are produced after said measured parameter R has converged upon the known value of the parameter R.
 14. A data processing system as set forth in claim 13 further characterized in that said parameter R is external standard ratio.
 15. A data processing system as set forth in claim 13 further characterized in that said fourth means comprises means for reducing the light input to said sixth means.
 16. The method of simulating quenching for a sample containing at least one radioactive isotope disposed in a liquid scintillation medium comprising the steps of a. measuring a selected quench correlation parameter for the sample, b. establishing a controlled variable simulated quench modulating signal having a a value which is a function of the measured quench correlation parameter, and c. employing the controlled variable simulated quench modulating signal to affect counting statistics.
 17. The method of simulating quenching for a plurality of samples, each containing at least one radioactive isotope disposed in a liquid scintillation medium, comprising the steps of a. measuring a selected quench correlation parameter for each of the samples, b. establishing a controlled variable simulated quench modulating signal having a value which is a function of the measured quench correlation parameter for each of the samples, and c. employing the controlled variable simulated quench modulating signal to reduce light inputs to the light transducers employed for measuring the correlation parameters by an amount sufficient to simulate quenching for all samples irrespective of their true internal quench level whereby the effective quench levels for all samples are the same and are somewhat below that of the most quenched sample in any given group of samples.
 18. The method of compensating for errors in computations of counting efficiency for n test samples containing one or more radioactive isotopes disposed in a liquid scintillator comprising the steps of a. establishing a fixed quench correlation parameter for which counting efficiency is accurately known and which is at least as low as the actual quench correlation parameter for the most quenched sample expected to be encountered, b. affecting counting statistics for each of said samples by creating a condition in which each sample can be counted as if it were quenched to a greater degree than it actually is, c. adjusting the created condition so that the measured quench correlation parameters for all of the samples converge to the fixed quench correlation parameter irrespective of the actual quench levels of any given one of the samples, and d. counting light scintillations created in each of the samples by interactions between radiations emanating from the isotopes therein and the liquid scintillating medium while the samples are subjected to the adjusted created condition.
 19. The method as set forth in claim 18 further characteRized in that the quench correlation parameter is net external standard ratio.
 20. The method as set forth in claim 18 further characterized in that the quench correlation parameter is net external standard count.
 21. The method as set forth in claim 18 further characterized in that the quench correlation parameter is channels ratio.
 22. The method as set forth in claim 18 further characterized in that counting statistics are affected by reducing the light input to the light transducers employed for counting light scintillations.
 23. The method as set forth in claim 22 further characterized in that the light input to the transducers is reduced by incrementally moving the transducers away from each of the samples.
 24. The method as set forth in claim 22 further characterized in that the light input to the transducers is reduced by interrupting the passage of light photons from the scintillations to the transducers.
 25. The method of simulating quench conditions in a test sample containing one or more radioactive isotopes in a liquid scintillation medium comprising the steps of a. establishing a fixed quench correlation parameter for which counting efficiency is accurately known, b. positioning the sample adjacent the light sensitive face of a light transducer, c. counting light scintillations produced in the sample and determining the quench correlation parameter indicative of the actual quench level in the sample, and d. modulating the counting statistics of the sample/light transducer combination so as to simulate a quench condition for the sample whereby the composite effect of the actual quench level for the sample and the simulated quench level is an effective quench level which produces a measurable quench correlation parameter corresponding to the fixed quench correlation parameter.
 26. The method as set forth in claim 25 further characterized in that the counting statistics are modulated by reducing the light inputs to the sample/light transducer combination.
 27. The method of computing true activity levels for test samples by simulating quench conditions in each test sample and wherein each sample containing a radioactive isotope in a liquid scintillation medium, comprising the steps of a. establishing a fixed quench correlation parameter for which counting efficiency for the isotope is accurately known, b. positioning each sample adjacent the light sensitive face of a photomultiplier, c. counting light scintillations produced in each sample and determining the quench correlation parameter indicative of the actual quench level in each sample, d. modulating the counting statistics of the sample/photomultiplier combination so as to simulate a quench condition for each sample whereby the composite effect of the actual quench level for each sample and the simulated quench level is an effective quench level which produces a measurable quench correlation parameter corresponding to the fixed quench correlation parameter, e. counting light scintillations produced in each sample by the isotope contained therein while the counting statistics are modulated to determine net counts per unit time, and f. dividing the determined net counts per unit time by the counting efficiency associated with the fixed quench correlation parameter.
 28. The method as set forth in claim 27 further characterized in that all of the samples irrespective of their actual quench level have superimposed thereon a simulated quench level sufficient to reduce their effective quench level to a point corresponding to the most quenched sample encountered in any given group.
 29. The method as set forth in claim 27 further characterized in that counting statistics are modulated by reducing the light input to the sample/photomultiplier combination. 